Apply slp vectorization on fully-vectorizable tree of height 2

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191852 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -311,6 +311,10 @@ private:
   /// \returns a vector from a collection of scalars in \p VL.
   Value *Gather(ArrayRef<Value *> VL, VectorType *Ty);
 
+  /// \returns whether the VectorizableTree is fully vectoriable and will
+  /// be beneficial even the tree height is tiny.
+  bool isFullyVectorizableTinyTree(); 
+
   struct TreeEntry {
     TreeEntry() : Scalars(), VectorizedValue(0), LastScalarIndex(0),
     NeedToGather(0) {}
@@ -917,15 +921,28 @@ int BoUpSLP::getEntryCost(TreeEntry *E) {
   }
 }
 
+bool BoUpSLP::isFullyVectorizableTinyTree() {
+  DEBUG(dbgs() << "SLP: Check whether the tree with height " <<
+        VectorizableTree.size() << " is fully vectorizable .\n");
+
+  // We only handle trees of height 2.
+  if (VectorizableTree.size() != 2)
+    return false;
+
+  // Gathering cost would be too much for tiny trees.
+  if (VectorizableTree[0].NeedToGather || VectorizableTree[1].NeedToGather) 
+    return false; 
+
+  return true; 
+}
+
 int BoUpSLP::getTreeCost() {
   int Cost = 0;
   DEBUG(dbgs() << "SLP: Calculating cost for tree of size " <<
         VectorizableTree.size() << ".\n");
 
-  // Don't vectorize tiny trees. Small load/store chains or consecutive stores
-  // of constants will be vectoried in SelectionDAG in MergeConsecutiveStores.
-  // The SelectionDAG vectorizer can only handle pairs (trees of height = 2).
-  if (VectorizableTree.size() < 3) {
+  // We only vectorize tiny trees if it is fully vectorizable.
+  if (VectorizableTree.size() < 3 && !isFullyVectorizableTinyTree()) {
     if (!VectorizableTree.size()) {
       assert(!ExternalUses.size() && "We should not have any external users");
     }